When you think of bacteria, you likely think about illness or some other negative impact of these microorganisms. Did you know that in soil bacteria play a huge factor in plant health? Many different species of bacteria live in the soil, and researchers classify these microorganisms into different groups based on their activity.

Researchers divide soil bacteria into four different groups based on their function in soil. Most examples of microorganisms in soil are ​decomposers​. The decomposer bacteria in soil feed on dead organic matter such as decaying plants and animals. Fungi, such as mushrooms, are another example of a decomposer. These types of bacteria, and all decomposers, play a vital role in breaking down nutrients and recycling them back into the food chain.

The next group of bacteria, the ​mutualists​, work together with plants so that both creatures benefit. Many types of soil microbes provide this benefit to plants, and one common function they serve is nitrogen fixation. However, not all microbes benefit plants.

One group of soil bacteria, known as ​pathogens​, can cause common ailments or deformations in plants. Finally, the ​lithotroph​ group utilizes different forms of nutrients than other bacteria, aiding in nitrogen cycling.

You can find many different examples of microorganisms in soil, and most benefit the ecosystem in some way. Some of the most beneficial soil bacteria help plants utilize nutrients that they otherwise wouldn't be able to intake with their roots.

Perhaps the most well-known and studied type of soil bacteria, ​nitrogen-fixing bacteria​ assist many different types of plants with nutrient uptake. The bacteria form a symbiotic relationship with the plant, taking up residence in the roots and converting nitrogen from the air into a useable form for the plant. In exchange, the plant provides the bacteria with carbon compounds.

Another important type of bacteria, ​actinomycetes​, work as decomposers for compounds that other bacteria and fungi have a hard time breaking down. This bacteria in dirt gives it the characteristic 'earthy' scent when you dig in your garden. Scientists also use some members of this group of bacteria to produce antibiotics.

Finally, ​nitrifying bacteria​ and ​denitrifying bacteria​ help convert different forms of nitrogen into useable forms. For example, nitrifying bacteria convert ammonium into nitrate, and denitrifying bacteria convert nitrate into nitrous oxide.

Researchers separate the many types of soil bacteria into taxonomic groups known as genera (singular: genus). Within these genera, an immense number of species exist. Some of the more common soil bacteria, by genus, are:

• ​Rhizobium​
• ​Bacillus​
• ​Mycobacterium​
• ​Streptomyces​
• ​Xanthomonas​
• ​Arthrobacter​
• ​Micrococcus​
• ​Pseudomonas​
• ​Corynebacterium​
• ​Agrobacterium​
  
• ​Flavobacterium​
  
• ​Alcaligenes​
  
• ​Clostridium​
• ​Azospirillum​

As decomposition experts, some types of soil bacteria naturally help break down common pollutants that impact our ecosystems. Researchers can use these bacteria to help break down excess pesticides and other contaminants, preventing potential runoff that could result in water pollution.

In fact, scientists are still finding different bacteria that could potentially help us combat pollution. Cornell scientists recently discovered a new species of soil bacteria, ​Paraburkholderia madseniana​. They named the microbe after their late microbiology professor, Gene Madsen. The bacteria effectively breaks down chemicals released during the burning of coal, garbage, oil and gas. Many of the chemicals that the bacteria broke down have been indicated as potential cancer-causing agents.

Despite the negative connotation that bacteria often receive, their microbial activity in soil provides numerous benefits to plants, entire ecosystems and even directly to mankind through their use in medicine and the mitigation of pollution.